1. Field of the Invention
The present invention relates to a wheel bearing device of a vehicle such as an automobile.
2. Description of the Related Art
Wheel bearing devices are generally classified into those for drive-wheel applications and driven-wheel applications. A drive wheel bearing device commonly comprises a wheel hub and a double-row bearing, or a unit of a wheel hub, bearing, and constant velocity joint.
One of the drive wheel bearing device designs has a construction in which one of the double-row inner races of the bearing is formed on an outer periphery of the wheel hub, while the other one is formed on an outer periphery of an outer joint member of the constant velocity joint. In this type of bearing device, the wheel hub and constant velocity joint need to be joined together to achieve relative positioning of the double-row inner races and to maintain certain preload applied to the inside of the bearing. There has been a new method of joining them by swaging as shown, e.g., in Japanese Patent Laid-Open Publication No. 2001-18605, which is considered advantageous in terms of effective use of axial space of the bearing and enhancement of the joint rigidity. With this method, a stem part of the outer joint member is expanded radially outward by plastic deformation so that it firmly engages with irregularities formed in the inner periphery of the wheel hub.
With such swaging, the portion of the stem part that is expanded need to have lowered hardness, so as to achieve good operability of the swaging and to secure sufficient joint strength. On the other hand, some parts of the outer joint member such as seal lands or portions that make contact with seal lips and inner races on the outer peripheral surface, and track grooves formed in the inner peripheral surface of the outer joint member need to have high hardness for securing fatigue durability.
Thus the outer joint member in the above swaged design needs to be formed with high-hardness portions and low-hardness portions. However, forming one member with varying hardnesses requires complicated fabrication processes, leading to higher costs. For example, if case hardening steel or carburized bearing steel such as SCM420 or the like is used for the base material of the outer joint member, its hardness can be increased by carburizing quenching after forging the steel into the shape of the outer joint member. In this case, the stem part needs to undergo decarburizing treatment and annealing after hardening so as to decrease its hardness, and the cost is accordingly high.
High carbon chromium bearing steel such as SUJ2 or the like may also be used for the sake of longer rolling life, but it will be difficult to smoothly expand the stem part radially outward because of the high material hardness even before the heat treatment. Moreover, since the outer joint member is most commonly formed by forging, the high carbon chromium bearing steel having high hardness is not suitable for its base material.
Furthermore, both bearing steels noted above contain expensive elements such as chromium and molybdenum, and the high cost of steel material itself will raise the overall cost of the wheel bearing device.